There is well known a vehicular drive system which is provided with at least one electric motor and a helical gear portion which are accommodated within a casing consisting of a plurality of casing members fixed to each other. Patent Documents 1 and 2 disclose examples of such a vehicular drive system in the form of a drive system control unit and a hybrid drive system, respectively. In the drive system control unit 1 of Patent Document 1, a drive system casing 60 consists of three casing members, and accommodates an electric generator G (electric motor) disposed on a first axis A1, an electric motor M disposed on a second axis A2, and a counter gear mechanism T serving as a helical gear portion disposed on a third axis, as shown in FIG. 4. In the hybrid drive system 10 of Patent Document 2, a first motor/generator 14 disposed on a first axis O1, a second motor/generator 18, and a power distributing mechanism 16 serving as a helical gear portion are accommodated within a casing consisting of three casing members in the form of a second motor housing 56, a first motor housing 64 and a motor cover 66, as shown in FIG. 1.